Introduction

Background

Rural highways account for a very significant portion of the national highway system and serve many vital mobility purposes, such as:

• connectivity between major urban areas,
• access to rural recreational areas (e.g., mountains, lakes, oceans),
• access to special events held in rural areas (e.g., concerts, regional festivals),
• evacuation route for extreme events (e.g., natural disasters), and
• diversion of traffic from another disrupted route.

Despite the importance of rural highways, infrastructure funding to improve operations is often more limited for them versus heavily congested urban roadways. Thus, to ensure effective investment of such funding, it is essential for highway agencies to be able to identify locations of poor operations and consider appropriate mitigation measures. For this to be possible, an agency needs traffic analysis methods that allow them to examine short sections of highway (e.g., a passing zone, signalized intersection) not just individually, but also within the context of an extended length (many miles) of highway.

Rural highways, which often span distances of 20-60 miles between urban areas, may consist of segments with a variety of cross-section elements (two-lane highway, multilane highway, passing lane sections) as well as intersections with different traffic controls (signal control, stop-control, roundabouts with yield control). These highways are also usually more varied in horizontal and vertical alignment than urban roadways.

The HCM, the standard reference for traffic analysis methodologies, contains analysis methods for all the individual segments or intersections that may comprise a rural highway; however, it does not include a method, or guidance, for connecting the individual roadway segments into a connected, cohesive, facility-level analysis.

It is important to continue to extend the capabilities of the HCM analysis methodologies, particularly at the facility level, so that roadway design and traffic engineers have the analysis tools they need for performing accurate and comprehensive facility evaluations. Furthermore, this is consistent with the fact that drivers typically evaluate the quality of their trip over its entire length, not just in separate segments.

The objective of this research was to (1) develop reliability and level of service analysis methodologies for rural highway facilities, consistent with existing HCM methods and (2) develop a guide on the application of the analysis methodologies.

Project Deliverables

The two primary deliverables for this project are NCHRP Web-Only Document 392: Developing a Guide for Rural Highways: Reliability and Quality of Service Evaluation Methods and NCHRP Research Report 1102: Reliability and Quality of Service Evaluation Methods for Rural Highways: A Guide.

Guide

Purpose of the Guide

The Guide is intended to assist transportation agencies charged with monitoring, maintaining, and improving rural highways of regional or statewide importance. More specifically, it is intended to assist with the evaluation of rural highways in three areas:

• Motorized vehicle traffic operations per HCM analysis methods,
• Motorized vehicle traffic operations per probe vehicle data analysis methods, and
• Overview of alternative analysis methods, to the HCM, for bicycles and recommendations for future bicycle operations research needs.

With the heterogeneity of cross-section (i.e., roadway segment type) composition over such distances, often 20+ miles, and the disparate HCM service measures (density, follower density, delay) across these segment types, the process for performing an HCM facility analysis across the variety of contiguous segments contained within a rural highway facility is not necessarily straightforward. The Guide proposes an analysis framework for assessing the level of service (LOS) of automobiles along a rural highway. In addition to LOS, several other facility-level performance measures are presented along with discussion about the analysis context in which such measures are useful for evaluating overall traffic operations along the route.

While simulation is always an option for analyzing a stretch of rural highways, the level of effort would be high for typical rural highway distances considered for analysis. In some situations, simulation may be warranted, but the methodology described in the Guide would still be a good first step and may even be completely sufficient. This methodology would also be much more efficient for performing 'what-if' scenario testing, where relative differences in results are the primary concern.

Reliability analyses on freeways and arterials are typically based on HCM guidance on scenario generation and predictive reliability. However, because of the typically limited data availability on rural highways, the reliability analysis is focused on historical probe vehicle data and meant to be used in conjunction with the HCM automobile LOS methodology.

Demand for cycling in urban areas and on rural highways is on the increase; yet is not clear which analysis procedures are best suited to cover large rural highway facilities or statewide analyses. The Guide summarizes existing HCM analysis methods used for the bicycle mode, as well as two popular alternatives, Level of Stress and Bicycle Compatibility Index. In addition, it proposes recommendations for future bicycle operations research needs based on two qualitative surveys.

The Guide is intended to serve as a companion to the HCM.

Guide Scope and Limitations

The facility-level analysis is important in assessing current conditions along important corridors for people and goods movement. The analysis methodology is also useful for evaluating facility performance for situations where significant changes in traffic demand and/or capacity may occur, such as in the following scenarios:

• Evaluation of a rural highway route to handle potential evacuation traffic demand (e.g., forest fire, hurricane)

• Evaluation of a rural highway route to handle potential diversion traffic demand due to an alternative route being closed or restricted due to construction, an incident (e.g., truck rollover) or a natural disaster (e.g., landslide/avalanche, flooding)
• Evaluation of a rural highway route to handle short-term spikes in traffic demand due to recreational activities (e.g., weekend ski season, Labor Day weekend beach travel, concert/festival)
• Evaluation of a rural highway route to handle short-term spikes in traffic demand and/or heavy vehicles due to season-specific activities (e.g., crop harvesting in farming regions)
• Evaluation of a rural highway route to handle a large increase in traffic demand as projected to occur as part of construction of a large generator for the purpose of regional economic development (e.g., tribal casino, Amazon distribution warehouse).

The parameters of the scope for this project generally required that the developed LOS evaluation methodology make use of the existing analysis methodologies within the HCM. However, to facilitate the development of a facility-level evaluation methodology, it was necessary to develop a few new computational procedures, largely for the purpose of connecting/stitching together component pieces of highway into a single facility for evaluation purposes. Furthermore, some planning-level simplifications, such as the classification of terrain and the treatment of signal progression along an arterial, were implemented. Such simplifications were included to 1) reduce the segmentation process effort and/or 2) reduce the complexity of the calculation process where the return on such precision is minimized for the relatively long lengths of rural highway.

The Guide also introduces a method for evaluating rural highway operations with the use of probe vehicle data. Over the last decade, the spatial and temporal coverage of probe vehicle data available from third-party vendors has improved immensely. Many state agencies now pay for subscriptions to providers of such data and are making use of the data to supplement their traditional data sources (e.g., fixed-point sensors) for assessing and managing traffic operations on their roadways. These data generally consist of average travel times/speeds and correspond to a sample of the vehicles traveling along a given roadway segment; thus, a key limitation of this data source is that it does not include flow rate. Another limitation of the sampling approach is that the accuracy can be highly variable for low flow rate traffic conditions. An additional challenge to the data is the spatial unit used for reporting--often referred to as a traffic message channels (TMC). This TMC length usually ranges from approximately 0.6-2 miles, and the TMC boundaries do not necessarily match those used for other traffic analysis purposes, such as for segments as defined by the HCM. The temporal resolution of the measurements ranges from approximately 1-5 minutes. Currently, the quantity and quality of probe vehicle data is much greater for urban areas than for rural areas. This gap, however, will continue to narrow with time.

The automobile LOS methodology presented in the Guide is not intended to handle oversaturated traffic flow conditions. For multilane and two-lane highway segments, the HCM analysis methodologies do not include any mechanism to deal with traffic demand exceeding capacity. In some instances, short periods of demand exceeding capacity can be accounted for in the intersection analysis methodologies. The HCM should be consulted for further information on this topic.

Guide Organization

The Guide is organized into three parts.

Part I

The first part is focused on analysis methodology descriptions and consists of the following chapters:

• Chapter 1 - Introduction: This chapter provides an overview of the Guide’s purpose, scope, and limitations. It also discusses the format of the Guide and how the user community can contribute to the content. Further, the chapter provides a summary of the research behind the Guide.
• Chapter 2 - Rural Highway Level of Service for Automobiles: This chapter provides the methodology used to assess traffic operational quality and LOS for the automobile mode on rural highways.
• Chapter 3 - Automobile Travel Time Reliability: This chapter describes methods for quantifying travel time reliability, from a historical perspective, based on probe vehicle travel speed measurements.
• Chapter 4 - Bicycle Operations Analysis on Rural Highways: This chapter provides an overview of commonly used methods for assessing bicycle operations, recommendations for which methods are most appropriate for certain bicycle analysis situations, and recommendations for enhancements to the commonly used analysis methods.

Part II

The second part provides an overview of the component HCM analysis methodologies that are incorporated in the rural highway analysis methodology for automobiles. This chapter does not replicate the full content of the relevant HCM analysis methodologies, but rather summarizes the chapters and sections that are utilized within the rural highway analysis methodology. This material will be updated as necessary to reflect updates to the HCM.

Part III

The third part is focused on case studies using real-world routes to demonstrate the analysis methodology in the Guide. This material is contained in a separate part of the document to facilitate the inclusion of additional case studies more easily in the future.

NCHRP Web-Only Document 392: Developing a Guide for Rural Highways: Reliability and Quality of Service Evaluation Methods

The main purpose of NCHRP Web-Only Document 392 is to document the details of the underlying research efforts used to develop some aspects of the analysis methods provided in the Guide. NCHRP Web-Only Document 392 also includes a literature review and summary of current practices. This content is organized in appendices as follows:

Appendix A: Literature Review and Current Practices

Appendix B: Two-Lane Highway Passing Lane Segment vs Multilane Highway Segment

Appendix C: Arterial Signal Spacing for Coordination

Appendix D: Intersection Influence Area

Appendix E: Highway Facility LOS Calculations

Appendix F: Bicycle Analysis Practices/Needs for Rural Highways, Survey of Practitioners

Appendix G: Bicycle Quality of Service on Rural Highways, Survey of Bicyclists